Non Over Sampling DAC

[rf_2006]

Bonjour à Tous

Je me lance dans la construction d'un NOS DAC, un DAC non oversampler, c'est à dire pas de filtre numérique avant d'attaquer les convertisseurs, de grande marque pro comme LAVRY qui équipent les studios utilisent cette technique.

Dan Lavry n'est pas favorable au NOS:

"The older DA's (before around 1990) did not have upsampling, because the concept did not exist. The problem with no over sampling was a serious one, in fact there were 2 problems:

1. The signal out of a DA is made of "steps". It is in fact the desired analog signal but with some added high frequency energy (the "steps") which is all above Nyquist, but needs to be removed. True, we do not hear such high frequencies, but sending such signals through electronic gear can cause problems. Say a power amplifier or headphone, or speakers are not designed to processes such high frequencies. The "step energy" covers a wide frequency range, from Nyquist to many MHz or even higher. So we want to remove (filter) that energy.
Now, say you have a 44.1KHz material ,ƒÏ Nyquist is at 22.0KHz. That calls for removing the energy above 22.05KHz, but keeping the audio (say up to 20KHz). It is very difficult to have a practical filter that does such a job. We have 2KHz of transition between all pass (without distortions and phase alteration) and full rejection.

2. Theoretical sampling is not a "stepped wave,ƒ˘, it is a very narrow pulse at sample time, but zero most of the time (zero between samples). But such signals have tiny energy so we resort to a practical wave ,ƒÏ the step wave (keep the value of a sample until the next sample). "The amplitude vs. frequency response of a "stepped wave,ƒ˘ has some loss as you approach Nyquist. That is undesirable, and difficult to compensate for.

Therefore, increasing Nyquist is a good thing. It makes a good filter design practical, and the flatness response to say 20KHz improves dramatically.

For that reason, all modern DA's I know have some up sampling. When you up sample to by say X16, only around 6.7% of the data is original, and the rest 93.4% is computed based on the original samples. Some modern sigma delta DA's work at much higher rates (X64 to X1024fs) but that is a different story, and the reasons for it are about noise shaping, but one should not get confused - sigma delta also start the process with some up sampling, then comes the noise shaping.

One can do a fine job of up sampling, or a poor job of it. But I would not even think of working at 44.1KHz and 48KHz without upsampling first.

As a rule, DA's do it (upsample) internaly. But say your DA can recieve 44.1KHz and 88.2KHz by design, then the designer can take a 44.1KHz signal and do a X2 upsampling before the DA (thus feeding the DA 88.2KHz). There is no reason to do it if the DA has a good internal upsampler. You can do it if the DA internal upsampler is poor. That is an implementation issue. "

http://www.lavryengineering.com/lavry_fo...2222#p2222



"So the NOS DA is either a piece of very inferior gear, or it is an up sampling DA that some sales guy calls NOS while it is not.

No need to be confused. When you hear such baloney, just ignore it. It is the folks that tell you that a NOS is a way to go are very confused and un informed."

http://www.head-fi.org/t/437340/any-bene...st_5905139